The calculator categorizes the severity of heart failure using the New York Heart Association (NYHA) functional classification system. It does this by analyzing a patient's reported physical abilities related to cardiac disease and matches them to one of the four NYHA classes.

Here's the breakdown of each class:

- Class I involves patients who have cardiac disease but do not experience any limitations in physical activity. Regular activities do not cause undue fatigue, palpitation, or shortness of breath.
  
- Class II fits patients who have a slight limitation in physical activity due to cardiac disease. They feel comfortable when at rest, but ordinary physical activities might lead to fatigue, palpitation, or breathlessness.
  
- Class III describes patients with a marked limitation in physical activity. These individuals are comfortable when resting, but even less than ordinary activities cause fatigue, palpitations, breathlessness, or chest pain.
  
- Class IV covers patients who cannot engage in any physical activity without experiencing discomfort. They exhibit symptoms of heart failure even when at rest, and any physical activity worsens these symptoms.

To determine the appropriate class for a patient, the calculator compares the user’s description of their physical ability with these four established patterns. Each condition is linked specifically to one of the NYHA classes, providing a clear pathway from symptom description to classification. For example, if a patient’s physical condition matches the criteria for Class I, then the result is "Class I." This comparison method efficiently identifies the level of heart failure severity based on self-reported abilities and symptoms.

The NYHA functional classification for heart failure, or New York Heart Association Functional Classification, is a scoring system used to quantify the severity of symptoms in patients with heart failure. This classification system is applicable to patients diagnosed with heart failure and is used to assess their functional capacity and limitations during physical activity. 

The NYHA classification is divided into four classes: Class I (no symptoms and no limitation in ordinary physical activity), Class II (mild symptoms and slight limitation during ordinary activity), Class III (marked limitation in activity due to symptoms, even during less-than-ordinary activity), and Class IV (severe limitations, experiences symptoms even while at rest).

The clinical utility of this calculator is to guide treatment decisions, assess prognosis, and evaluate the effectiveness of therapeutic interventions in patients with heart failure. It is also commonly used in clinical trials to stratify patients and measure outcomes. 

There are no specific exclusion criteria for its use, but it is subjective and relies on patient self-report, so it may not be as accurate in patients with cognitive impairment or communication difficulties.